Can't figure out why this looks so grainy
These are the fastest ways.
If you are not in Cycles Render then switch to Cycles and on the Render Layer tick mark Denoising.



If that does not give you the desired effect then increase the Render Samples as well , at the Sampling section, but will increase your render time.



These two settings usually solve the problem.

Sometimes I find that unticking the Reflective Caustics and the Refractive Caustics will improve some scenes. These settings are under the Light Paths settings under Sampling settings.